1. Field of the Invention
This application relates to combustion of gaseous fuels in a manner which meets today's pollution requirements and, more particularly, to a burner and method for producing a low temperature flame utilizing excess combustion air or flue gas recirculation.
2. Description of the Prior Art
Nitrogen oxide (NOx) emission regulations applied to combustion processes are becoming increasingly more stringent. Benchmarks for these regulations are frequently set by the Southern California Air Quality Management District ("SCAQMD"), which has promulgated regulations that would limit the NOx emissions from burners operating with natural gas to less than 25 parts per million on a volume basis ("ppmv"), corrected to 3% oxygen. Other states have enacted or are contemplating similar legislation.
All combustion reactions produce NOx via one of two mechanisms. Thermal NOx is produced in high temperature flames by fixation from nitrogen and oxygen present in the combustion air. Fuel NOx is produced from chemically bound nitrogen present in the fuel combusted. Depending on the nitrogen concentration present, fuel NOx generation rates can be orders of magnitude greater than thermal NOx generation rates. This invention is directed to reducing thermal NOx only. The generally accepted mechanism of thermal NOx formation is described by the following reaction equations: EQU N.sub.2 +O.revreaction.NO+N (1) EQU O.sub.2 +NO.revreaction.O (2)
The forward reaction rate constant for reaction (2) is much larger than the corresponding rate constant for the forward reaction of equation (1). Therefore, a cursory analysis might lead to the conclusion that reaction (2) is the dominant reaction producing NOx.
However, the concentrations of the species involved in the reactions must also be considered. The nitrogen and oxygen are produced by the thermal disassociation of N.sub.2 and O.sub.2 at elevated temperatures. Molecular nitrogen is thermally disassociated at a much slower rate than oxygen. This results in a large population of oxygen atoms early in the reaction while the nitrogen atom population remains relatively small. This high concentration of oxygen relative to nitrogen is sufficient to offset the disparity in rate constants between reactions (1) and (2).
Reducing the peak flame temperature in a burner is a well established method of reducing the NOx generation rate. Tests have confirmed a direct relationship between equilibrium oxygen mole fractions and equilibrium NO mole fractions present in the reactions taking place during combustion of natural gas. It has been established that equilibrium oxygen mole fractions are much lower below 2500.degree. F., with the consequence that NO mole fractions are also lower below this temperature.
There are two possible methods of reducing flame temperature in a burner. One extracts radiant heat from the flame by transfer to cooled surfaces surrounding the flame. There are practical limitations to this technique, however. The loss of heat radiation from the center of the flame will be screened by the gases surrounding the center. The outermost gases successfully radiate their heat to the cooled surfaces, but the central gases only radiate to the gases immediately surrounding them. Therefore, the reduction in maximum flame temperature is not uniform and ineffective.
The second method of reducing the flame temperature is by introducing a sensible heat load to lower the temperature. This is the principle behind flue gas recirculation, which also reduces the oxygen concentration in the flame envelope. The flame temperature will also be moderated by using high excess air levels.
Prior efforts to achieve low flame temperatures and reduced NOx levels have exposed several problems. Particularly, it can be difficult to maintain stable combustion near the lower flammability limit of a given fuel when the flame temperature is low. Additionally, flameouts and high carbon monoxide emission levels can occur. It has been found that almost perfect mixing of fuel and oxygen prior to combustion is essential to achieving the lowest NOx levels without these problems, particularly using single stage burners. The problem of burner flashback becomes a consideration when fuel and oxygen are premixed before ignition.
Therefore, it is an object of the present invention to minimize thermal NOx generation when combusting fuels which contain negligible amounts of fuel bound nitrogen. It is a further object to provide a burner and method which maintains stable combustion at low flame temperatures, and provides accurate mixing of fuel and oxygen in the flame to avoid flameouts and high carbon monoxide emissions. Finally, it is an object of the invention to provide a premix burner and method which meets today's stringent NOx standards, while eliminating the problem of burner flashback.